Hi

>         In this design, I tie the MCLR pin to VDD, and enable the WDT whose
> time-out period is set to be about 80mS, though VDD has a quite slow
> rise at power-on. My concept is that despite of the possible malfuction
> at the beginning, the WDT will can resume the correct opreation soon if
> account for this in programming. I don't know if I am right or not and I
> dont have much time to try this over and over.
>
No, I don't think its safe.  This was discussed some time ago on the list
when someone noticed that the watchdog did not reset the PIC correctly
after a brownout condition.  The PIC also seems to be able to go into a
state where even taking MCLR low does not correctly reset the PIC.

Niki